During the past decades there have been considerable developments within the fields of radiation therapy and medical diagnosis. The performance of external beam radiation therapy accelerators, brachytherapy and other specialized radiation therapy equipment has improved rapidly. Developments taking place in the quality and adaptability of radiation beams have included new targets and filters, improved accelerators, increased flexibility in beam-shaping through new applicators, collimator and scanning systems and beam compensation techniques, and improved dosimetric and geometric treatment verification methods have been introduced.
Furthermore, a number of powerful 3-dimensional diagnostic techniques have been developed, ranging from computed tomography (CT), positron and single photon emission computed tomography (PET and SPECT) to ultrasound and magnetic resonance imaging and spectroscopy (MRI and MRS). Equally important is the increased knowledge of the biological effect of fractionated uniform and non-uniform dose delivery to tumors and normal tissues and new assay techniques, including the determination of effective cell doubling times and individual tissue sensitivities, allowing optimization of the dose delivery to tumors of complex shape and advanced stages.
A major problem in the field of radiation therapy and diagnosis today is the movement of a patient on a patient couch during radiation therapy or diagnosis. The movement can be inevitable, such as the movement of the patient body caused by breathing, or the patient may slightly adjust his/her position or posture on the couch.
In volumetric imaging, including CT, PET, MRI, MRS or SPECT, the resulting images will contain artifacts, e.g. blurring and discontinuities, since the whole volume is not being acquired at a single respiratory phase/amplitude. These artifacts are due to, among others, the respiratory-induced movement of at least a portion of the patient body during image data acquisition.
In radio therapeutic treatment, the tumor and surrounding tissue will undergo displacement and deformation due to the respiration during the treatment delivery, and this will cause an uncertainty in the distribution of the dose delivered.
One solution to these problems is to monitor the patient's breathing and use this respiratory signal to “gate” the imaging or treatment, either prospectively or retrospectively. In prospective gating, the imaging or treatment process is controlled so that it will only be active during a specified respiratory gating window—when the phase or amplitude of the respiratory signal is within certain pre-defined limits. In retrospective gating, which is only applicable during imaging, the respiratory signal is captured in parallel with the volumetric imaging process and these are typically synchronized to a common time base. After the scan is completed, the respiratory signal is used as a key to sort the volumetric data parts acquired into different “buckets”, where each bucket contains volumetric data acquired during a specified respiratory gating window. This allows combination of several complete volumetric images, each from a specific respiratory amplitude/phase.
There are today several techniques available at the market for acquiring the respiratory signal. For example, the Philips CT gating system uses a belt that is fastened around the patient's chest. This belt is employed for measuring the chest displacement due to respiration. Varian uses another technique for respiratory movement monitoring, see for example document [1]. This technique involves placing a block with retro-reflective markers on the patient's chest. The movement of the chest due to breathing can then be tracked by an infrared (IR) camera with an integrated IR light source.